Fix some leaks in lld's YAML parser

[Pete Cooper via llvm-commits]

Hi Rafael

This should fix some of the leaks seen in PR 21466.

The cause was the MappingNormalizationHeap struct which was allocating atoms.  In the binary file parser we allocate atoms in the file::allocator() so that we can avoid the free cost later.  

This patch makes atoms use the file allocator in the YAML parser when needed.

Note, this doesn’t fix all the leaks.  There’s still a bunch I’m trying to understand, but its a good start.

Thanks
Pete

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